Turbochargers are widely used on internal combustion engines, and in the past have been particularly used with large diesel engines, especially for highway trucks and marine applications. In distinction to superchargers, which derive their power directly from the crankshaft of the engine, turbochargers are driven by the engine exhaust gases. Exhaust gases are directed to and drive a turbine, and the turbine shaft is connected to and drives the compressor. Ambient air is compressed by the compressor and fed into the intake manifold of the engine.
More recently, in addition to use in connection with large diesel engines, turbochargers have become popular for use in connection with smaller, passenger car power plants. The use of a turbocharger in passenger car applications permits selection of a power plant that develops the same amount of horsepower from a smaller, lower mass engine. Using a lower mass engine has the desired effect of decreasing the overall weight of the car, increasing sporty performance, and enhancing fuel economy. Moreover, use of a turbocharger permits more complete combustion of the fuel delivered to the engine, thereby reducing the hydrocarbon emissions of the engine, which contributes to the highly desirable goal of a cleaner environment.
Fixed geometry turbochargers are normally designed to operate at peak efficiency at a particular engine speed and load. Smaller passenger car engines are normally operated over a wide range of engine speeds and load. When a turbocharger is operated over a wide range of engine speed and load, the turbocharger components function outside the optimum design range and consequently suffer loss of efficiency that adversely affects engine performance. In operation of a turbocharger employing a radial compressor, it is known that pre-whirling the supply air to the compressor can have beneficial effects in both broadening the efficient operating range of the compressor and increasing its efficiency. Both positive and negative pre-whirl can be used to achieve these desired effects.
An early attempt at choking the flow of air entering a compressor is shown in U.S. Pat. No. 3,723,021 to Bartholomew. That patent shows a flow choking arrangement for the intake of an axial flow primary compressor such as are used in connection with an axial flow gas turbine engine. A flexible vane is fixed at its leading edge and is deflected sideways along the circumference of a cylindrical intake channel. The radially outer corner of the trailing edge of the vane is fixed to a rotating disk that is in turn pinned into a cylindrical ring in the outer wall of the channel. The rotating disk twists the vane, and the pin rotates within the ring, as the ring moves circumferentially. The rotating disk rotates to accommodate the reduced axial length of the vane in its deflected position. The radially outer edge of the vane is cut away to prevent interference with the wall of the cylindrical channel when the vane is deflected. This complicated arrangement permits limited deflection of the trailing edge of the vane, is prone to jamming and involves excessive friction to operate, and the offset attachment of the vane to the rotating pin permits deflection of the vane in only one direction. This system is very complicated, has many moving parts, and very limited utility. It appears to be a complex, expensive structure intended for use on a large expensive turbine engine in which pre-whirl in both positive and negative directions is not considered.
The benefits of pre-whirling supply air to a radial compressor in both a positive and negative direction were discussed more fully in a paper published in the Proceedings of the Institute of Mechanical Engineers, Vol. 189 43/75 (March, 1975), titled “Experimental and Theoretical Performance of a Radial Flow Turbocharger Compressor with Inlet Pre-whirl,” by Wallace, Whitfield and Atkey. In that paper, Wallace, et al. suggested that positive pre-whirl, that is, generating a pre-whirl of the intake air having a tangential component of velocity in the same direction as the impellor rotation, can broaden the operating range of a turbocharger by moving the surge line of the compressor map to a lower mass flow rate. The surge line of a compressor map defines the lowest flow rate at which a compressor can operate in a stable condition. At flow rates below the surge line, a compressor will vibrate violently and cease useful operation. Moving the surge line of a compressor to a lower mass flow rate permits stable operation at lower flow rates and engine speeds. This allows efficient and stable operation of a turbocharger at, for example, engine “lugging” speed. Wallace et al. also suggests that higher compressor efficiency can be achieved by imposing a negative pre-whirl at high operating speeds.
U.S. Pat. No. 3,922,108 to Benisek discloses an apparatus that generates positive pre-whirl by providing fixed vanes adjacent the compressor inlet. The Benisek device varies the volume of supply air that passes over those vanes by throttling a central unvaned passage with a butterfly valve, thereby varying the amount of pre-whirl supplied to the compressor.
In U.S. Pat. No. 4,780,055 to Zloch et al., a plurality of circular sections are pivotally arranged in the compressor intake channel to restrict flow therein. The sections are not intended to generate pre-whirl. In fact, Zloch et al. states that the flow channel is narrowed after the restrictor sections to generate an accelerated jet flow that suppresses wake disturbances from the circular restrictor sections to provide a uniform inlet flow to the compressor rotor disk.
Other patents showing a choking vane structure for modifying the inflow of gas to a radial compressor in a refrigeration device are U.S. Pat. No. 3,667,860 to Endress et al. and U.S. Pat. No. 6,039,534 to Stoner et al. Stoner shows two sets of fixed vanes, the first of which deflects the gaseous refrigerant flow a little, and the second deflects it more. In addition, U.S. Pat. No. 5,096,374 to Sakai et al. shows a choking mechanism for a turbo-compressor having a very complicated external circumferential ring linked to actuating shafts extending into the intake channel. None of these references mentions whether it is intended to generate, or is capable of generating, positive and/or negative pre-whirl to the compressor inlet.
Lastly, U.S. Pat. No. 5,560,208 to Halimi et al. refers to the Wallace et al. paper cited above and suggests that means should be provided on the compressor intake to switch from positive to negative pre-whirl, but no structure to accomplish this desired end is disclosed or suggested.